The first question about the Mongol conquests is: Why did
the Mongols erupt from Mongolia in the early 13th century
to begin their conquests of the rest of the world, creating
the largest contiguous land empire in world history? There
has been considerable speculation about the reasons for the
Mongol eruption from Mongolia, and though there is no scholarly
consensus on specific reasons, many have pointed to the causes
of ecology, trade disruptions, and the figure
of Chinggis
(Genghis) Khan.

Ecology
In the period from 1180-1220, Mongolia experienced a drop
in the mean annual temperature, which meant that the growing
season for grass was cut short. Less grass meant a real danger
to the Mongols' animals, and, since the animals were truly
the basis of the Mongols' pastoral-nomadic life, this ecological
threat may have prompted them to move out of Mongolia. [Also
see The Mongols'
Pastoral-Nomadic Life].

Trade Disruptions
A second reason often mentioned is the attempt by Mongolia's
neighbors in north and northwest China to reduce the amount
of trade with the Mongols. Since the Mongols depended on trade
for goods that they desperately needed  such as grain,
craft, and manufactured articles  cessation of trade,
or at least the diminution of trade, could have been catastrophic
for them. The attempts by the Jin dynasty, which controlled
North China, and the Xia dynasty, which controlled Northwest
China, to reduce the level of trade that the Mongols could
expect, created a crisis for the Mongols. Unable to obtain
goods that they so desperately needed, the Mongols' response
was to initiate raids, attacks, and finally invasions against
these two dynasties.

Chinggis Khan's Personal Mission
A third explanation has to do with Chinggis Khan himself,
in particular his shamanic beliefs. It is said that Tenggeri,
the sky god of the Mongols, gave Chinggis the mission of bringing
the rest of the world under one sword  that is, bringing
the rest of the world under the shamanic umbrella  a
mission that may have motivated Chinggis to begin his conquests.
Whatever the explanations, they all gravitate around the figure
of Chinggis himself. Thus it is important to see what Genghis'
policies led to and to analyze his life and career.